Regeneration of photographic fixing baths



Patented Sept. 8 1936 PATENT OFFICE REGENERATION OF PHOTOGRAPHIC' FIXINGBATHS Karl Kieser, Beuel-on-the-Rhine, Germany No Drawing. ApplicationJune 28, 1934, Serial No. 732,885. In Germany July 12, 1933 15 Claims.

render them suitable for re-use; this method comprising adding to such afixing bath a soluble thallous salt, such as thallous sulfate, andfiltering off the thallous iodide and bromide thereby precipitated; allas more fully hereinafter set forth and as claimed.

The recovery of silver from photographic fixing baths is Well known andis rather widely practiced. One of these methods consists in theaddition of a reducing agent, such as an alkali metal attempted tore-use a fixing bath de-silvered in accordance with one of thesemethods, difiiculties are encountered. It is found that the fixing-acetion of the bath is greatly reduced in speed. This is true even thoughthe strength of the fixing.

agent in the bath has been restored to its original value. a

The cause for the reduced speed of the fixing action of a de-silveredfixing bath is the accumulation of iodides and bromides in the bath. Ofthese the iodides appear to be the most deleterious. In commercialpractice a once-used 20 per cent sodium thiosulfate fixing bath whichhas been de-silvered rarely contains over 500 mg. of sodium or potassiumiodide per liter. This amount, while small, appreciably decreases thespeed of fixing and, if the bath is repeatedly desilvered after use, thelength of time required for the fixing operation soon becomesprohibitive. It is believed that the accumulated iodides and bromidesproduce this decreased speed of fixing by decreasing the solubility ofthe corresponding silver salts in the bath.

No methods have been suggested previously in the art for the removal ofthe iodides and bromides from used fixing baths. I have found that thismay be readily accomplished by a simple precipitation procedure whichconsists in adding to the bath a soluble thallous salt followed byfiltering off the precipitate. I have found that this precipitation willoccur even in strong solutions of sodium thiosulfate and in the presenceof neutral or acid sulfites, alum, or chrome alum, or of other commonadditions to fixing baths.

The removal of the iodides and bromides is almost as complete as frompure aqueous solutions of the iodides and bromides. The bromides are notremoved as completely as are the iodides but are less objectionable. Thequantity of bromides remaining in a fixing bath after the addition ofslightly more than the calculated quantity of a thallous salt appears tohave no appreciable eifect upon the speed of the fixing action. Theiodides are removed almost quantitatively by the addi-,' tion of athallous salt in combining proportions. The yellow precipitate formedsettles readily and can be easily filtered off. This addition may, ofcourse, be made prior to the de-silvering of the fixing bath. Or theprecipitation with a thallous salt can be accomplished simultaneouslywith the de-silvering of the bath. Advantageous ly,however, the thalloussalt is added after; the

bath has been de-silvered; the procedure being essentially the same ineach of the three cases.

This permits the recovery of the thallium from the mixture of its iodideand bromide which is obtained as a'precipitate. When the thallium isrecovered in this manner my method can be operated very economicallyeven at the present prices. of thallium salts.

1 In a specific embodiment of the present invention I first de-silvered100 liters ofa once-used fixing :bath. by theaddition of sodiumsulfideand sodium hydrosulfite. The resulting precipitate was filteredoff. The filtrate had a sodium iodide content of about 450 mg. perliter, as estimated from an analysis of its silver content made prior tode-silvering and from the known iodide content of the photographicemulsion. I added to this filtrate while stirring 100 grams of thalloussulfate dissolved in 2 liters of water. There was an instantaneousprecipitation of a fine crystalline yellow sediment which settledquickly. This was removed by filtration. This precipitate was found tocontain practically all of the iodides of the bath and a largeproportion of the bromide content. The filtrate was reconstituted by theaddition of sodium thiosulfate and its original content of sodiumbisulfite was renewed by the addition of this material. The bath wasthen employed as a fixing bath and it was found that it was the fullequivalent of a freshly prepared bath. The rate of its fixing action wasfully restored. The thallium content was then recovered from theprecipitate of iodides and bromides by gentle heating with a slightexcess of concentrated sulfuric acid. In this operation the iodine andbromine evolved were recovered. The thallous sulfate recovered in thismanner was employed in a repetition of the above process.

sidered as being only slightly soluble, such as the sulfite, dithionate,thiosulfate and orthophosphate, can be used. Such salts areadvantageously added in finely divided form and usually as a magma or afine aqueous suspension. When employed in this manner any thallous salthaving a solubility somewhat greater than thallous bromide can beemployed, such salts requiring only a short time to react with and toprecipitate the iodides and bromides. When thallous thiosulfate isemployed in this manner, sodium thiosulfate is formed by metathesis andthis partially reconstitutes the bath so far as this component isconcerned. Other thiosulfate fixing baths can be treated by my newprocess, the alkali metal and ammonium thiosulfate fixing baths beingex.- amples. The salts commonly added to such fixing baths do notinterfere. Other modifications falling within the scope of the appendedclaims will be evident to those skilled in the art.

What I claim is:

1. In the process of regenerating photographic fixing baths, the stepwhich comprises adding to such a bath a soluble thallous salt.

2. In the regeneration of photographic fixing baths, the process whichcomprises adding to a used fixing bath a soluble thallous salt andfiltering ofi the resulting precipitate.

3. The process of claim 2 wherein the thallous salt is added to thefixing bath prior to the desilvering of the bath.

4. The process of claim 2 wherein the thallous salt is added to thefixing bath after the bath has been de-silvered.

salt is added to the fixing bath while the bath is being de-silvered.

6. The process of claim 2 wherein the thallous salt is thallous sulfate.1

7. In the regeneration of photographic fixing baths, the process whichcomprises adding to a used fixing bath a soluble thallous salt,filtering off the precipitate, recovering the thallium content of saidprecipitate as a thallous salt and adding the recovered thallous salt toa second used fixing bath.

8. The process of claim 7 wherein the thallous salt is thallous sulfate.

9. The'process of claim 7 wherein iodine and bromine are also recoveredfrom the said precipitate by heating with sulphuric acid.

10. In the process of regenerating used thiosulfate fixing baths, theprocess which comprises adding to such a bath a thallous salt having asolubility greater than that of thallous bromide and removing theresulting precipitate.

11. The process of claim 10 wherein the fixing bath contains athiosulfate selected from a group consisting of the alkali metal andammonium thiosulfates.

12. The process of claim 10 wherein the added thallous salt is selectedfrom a group consisting of the sulfate, acetate, carbonate, nitrate,sulfite, dithionate, thiosulfate and phosphate.

13. In the process of regenerating used thiosulfate fixing baths, theprocess which comprises adding. to such a bath a slightly solublethallous salt in the form of an aqueous suspension and removing theresulting precipitate.

14. In the process of regenerating used thiosulfate fixing baths, theprocess which comprises adding to a used sodium thiosulfate fixing batha quantity of thallous sulfate sufiicient to precipitate the iodides andthe bulkof the bromides, filtering off the precipitate, treating theprecipitate. with sulfuric acid to obtain thallous sulfate and employingthe so-recovered thallous sulfate in a repetition of the cycle.

15. In the process of regenerating used fixing baths, the steps whichcomprise precipitating substantially all the iodides and the bulk of thebromides from the bath by metathetical, reaction with a thallous salt,and filtering ofi the precipi- 5. The process of claim 2 wherein thethallous tate.

KARL KIESER.

